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The truth about air intakes and filters

These reports, based on fairly well controlled measurements:


  1. Aftermarket “high flow” and oiled filters do not work better than conventional and OEM filters on stock engines.
  2. Air intake and filter restriction has virtually no effect on MPG in modern cars and very little effect on a 1972 carbureted car.
  3. Modern intake systems have very little restriction leading up to the filter,
    so the claims of the cold air intake and short ram intake makers are mostly bogus on stock engines.

These will be good links to throw out there when someone claims their K&N filter makes their Camry run like a Corvette.

While I generally agree with circuitsmith’s comments there is one area that filters may make a noticeable difference.

The area is limited to highly modified cars. It is interesting that a exception to the general rule generally only exist with those highly modified cars. This in turn tends to give the false image that the filters (or lack of filters) will make a difference to an otherwise un-modified car.

Let me share something of my younger brothers has a 2000 something Taurus,it had a cracked exhaust manifold on one bank-you could not hear the exhaust leak until he installed a low restriction air filter element in it-Kevin

Years ago I had access to a dynometer near the shop where I worked. I used it to test vehicles after engine modifications or exchanges and came to this conclusion. The engine horsepower and torque readings were the same when using the customers K&N filter, the stock filter or no filter. I use OEM filters only because nothing better has come down the pike. If you listen to advertising hype you will find they all show an improvement of one type or another. It’s still bogus.

I wholeheartedly agree that these K&N CAI intakes and their “high flow” filters are, well, ineffective at best.
I also support the thought that aftermarket filters make no difference on gas mileage, as the .gov report found.

However, I have a different perspective on ram air. Ram air can make a very clear difference on acceleration under WOT above 50 mph if done correctly, and if the injectors are up to it.Most stock injectors do have some excess capacity to prevent saturation. I’ve installed what’s basically a ram air system in my own car, and it makes a very clear difference.

It’s important to realize that I’ve gained power when at speed and at WOT. Cruising is unaffected, and gas mileage cruising is unaffected. I’m simply able to push more air in at WOT when there’s a bit of pressure on the borrom side of the filter.

Ram air does wotk. But for power, not for mileage. As a matter of fact, if you use the added power opportunity you’ll discover that mileage will drop a bit. Thyat added power comes from the ability to use more gas…that’s where the power resides.

NOTE: I also removed a carbon filter from the airbox, which had I left it there probably would have stifled the added airflow.

I agree with mountainbike on his “ram air” statement. Ram air is effective for power. A turbocharger is better and a full supercharger is the best. I won’t mention nitrous oxide. A turbocharger and a supercharger is just ram air on steroids.

Most “short ram intake” kits aren’t a true ram air system. They’re just short lengths of pipe that end under the hood.

This is typical of the junk on ebay:

You’re right of course circuitsmith.
I love the hype in the ad. How can replacing the plastic/rubber piping increase air convection/flow rate and add 5-10 HP? Oh that’s right. It can’t. Bogus!

  • This short ram racing air intake system replaces your stock plastic/rubber piping
  • Increases the air convection/flow rate
  • Adds instant horsepower gain (~5-10 hp)

You are absolutely 100% correct Circuitsmith. This stuff is junk.

Missileman, that’s true. The nice thing about a mild boost with a true ram air intake is that it generally doesn’t require new injectors and remapping. On the other hand, it does nothing “off the line”. You gotta get the air comin’ in. And it can be done on the cheap. I made my own system for about $40.

Thanks Circuitsmith. It’ s good to reinforce bargin basement after markets that if indeed worked, would be standard equipment.

Where is the entrance point for your ram air system SMB?

If you doubled the size of your mouth, would you be able to run faster?

What if we also doubled your lung capacity and fitted you with those fancy carbon fiber spring shoes?

Point being, you have to address the system as a whole. One part taken out of a systematic approach is not going to have much effect.

Some of these parts like the oiled air filters were developed for specific purposes and when that market was saturated, manfrs looked for other areas to exploit. I can’t fault them for puffing and luring in the ill-informed.

My car picked up 5 RWHP using a K&N filter vs. a stock style paper filter when it was being dyno tuned after the supercharger install. The runs were done within ten minutes of each other The only cavet here is that the paper filter probably had about 5 or 6k miles on it.

With some trepidation I will share a modification I recently made on the intake system of my car.
I posted this on a couple of Toyota Corolla & Matrix forums and got responses that ranged from positive to dogmatic:

I re-routed the air intake on my 1zz to draw in cooler air.
I wanted to disturb the stock intake (which is well designed, IMHO) as little as possible.
I didn’t want to spend much $$ (ended up costing under $10), didn’t want to make more noise, didn’t want to fiddle with the MAF sensor and didn’t want to deal with the little cone filters or oil impregnated filters, which let more dirt in.
And it’s a 10 minute job to go back to stock.

Whatever restriction the stock system has is only significant at wide open throttle and high rpm, a condition I basically never reach.
What I want is more mid-range torque, especially on a hot day with the AC cranking.

The stock system ended in front of the battery with some cool air coming in through the gap around the headlight housing.
I removed the end piece, bent the fiber hose down and loosely fitted a length of 3" “dryer vent” aluminum flex hose.
This hose is routed through a gap behind the fog lamp mounting point and ends near the lower grill.
Now the air is taken from a point before the radiator can heat it.
The loose fitting at the fiber hose, plus the end point out of the incoming air stream should reduce any danger of hydrolock.
I decided not to point the inlet facing out the grill for a “ram” effect because it might take in too much water in heavy rain.

Here’s an OBDII chart with the original intake showing coolant temp, speed and intake temp on a 35 minute drive.

This was a cold start, ambient temp 50F, foggy using AC & defroster, mostly stop and go 35mph and below.
Intake stays cool until ~10.5 minutes when I’m waiting in line at the recycling/trash dump.
The radiator gets fully warmed up and the intake rises to 15-20 deg. above ambient and stays there even after I get going again.

With the warm weather on the East Coast I’ve been unable to do a similar drive with the modified intake.

The above is a warm start, ambient 81F, again mostly stop and go under 35mph.
The intake is heat soaked but cools off to within a few degrees of ambient in ~5 minutes.
With no AC use the radiator fan doesn’t kick on until the coolant gets hotter: 205F vs 188F.
The intake stays pretty cool until I get into some bogged down in 10mph traffic at ~54 minutes.
Once I get going the intake starts cooling down. In the meantime the radiator is fully hot & fan is cycling.

By the seat of the pants the throttle response didn’t get sluggish in the hot stop-and-go like it has in the past.

Here’s 30 minutes of another run.
Driving through town then on to the highway.
Warm start, ambient 61F.
The intake falls to within 2 deg. of ambient after a few minutes on the highway.

Caddyman, in my case my car had two false ducts in the front bumper with false grills in them. The openings are behind tapered sides, as if to produce a venturi effect (increased velocity/greater density). I removed one of the false grills, replaced it with a real grill that I manufactured, created a plastic box behind the opening using plastic from a storage bin and automotive mounting tape, and ran two shop vac hoses from the top of the box to the filter box intake.

Now my car has one false duct and one functional duct.

Who stole my EDIT button!

Circuitsmith, I dealt with the water ingestion concern by running the air directly into a plastic box with two small drain chutes in the bottom, and taking the air for the intake out of the top of the box. The concept is that larger drop of the water, having greater inertia, will continue past the air being drawn up and to the back of the box, where it’ll then drain down and out. I tested the concept extensively, and it works like a charm.

There’s a little gear shaped button to the right of your name, slightly to the right of the “like” buttons.
Click on it and the “edit” button magically appears.

Ah, cool. Thanks for the heads up. The gear must represent a doubleclutch.

Here’s another air filter study:

TwinTurbo: “If you doubled the size of your mouth, would you be able to run faster?”

Evidently, Breathe-Right Nasal Strips help athletes breathe better, unless it’s also a scam.